The pharmacokinetics of 4-acetyl tritium vinblastine in two patients

Vinblastine, labeled with tritium in the 4-acetyl group, was given to two patients with malignant disease, and the pharmacokinetic behavior of the drug was determined. Clearance of radioactivity from the blood was biphasic, with t1/2 values for a first rapid phase of 4.25 and 4.78 min, and for a slower phase of 185 and 195 min. The volume of the central compartment was calculated as 29.7 and 39.4 liters, while the total fictive volume of distribution was 86.4 and 111.4 liters. Binding to blood components occurred in the order: plasma greater than platelets greater than red blood cells greater than white blood cells. Excretion of radiolabel occurred via the stool and the urine so that, after 72 hr, 25 and 41% of the total dose had appeared in the former and 19 a nd 23% had appeared in the latter. Appreciable amounts of unchanged drug appeared in the urine, while very little appeared in the stool, suggesting hepatic metabolism, consistent with prior animal studies.